The Brief: Sis can be an abbreviation for sister, a term of familiarity, or a ubiquitous filler-word.

DEEP DIVE
Sis can be a feminine version of the words “bro” or “dude.” Like the word dude, it can be added into a conversation at random times. The term sis has origins in communities of black women and has since become a widespread term that can be used by and refer to anyone.
sis, it’s time for you to be happy again.
— daij 🌁 (@simplydaij) November 12, 2018
The way he's looking at her in this picture…sis drop this potato and go. https://t.co/qRKS06Tyj1
— ❄Mikki Kendall❄ (@Karnythia) November 12, 2018
Oh my God leave us alone sis https://t.co/YGudMx3Jmt
— George M Johnson (@IamGMJohnson) November 12, 2018
Go off, sis
— Liza✨ (@LizaaaDaphne) November 12, 2018